The present invention relates to a travel guiding apparatus for vehicle, which is capable of guiding a vehicle by searching a course from a starting point to a target point on a road map shown on a display screen.
The Japanese laid open patent publications No.133600-87 and No. 173297-89 disclose such a vehicle travel guiding apparatus which is capable of setting any start and target points on a road map shown on a display screen and searching digital map data to find an optimal route therebetween for the purpose of guiding the vehicle.
The digital map information contains data on positions and connections of nodes N of line segments which, as shown in FIG. 4, approximately represent a figure of actual mapped roadways shown in FIG. 3.
A prior method for searching a course on the basis of the digital map data is such that nodes on a road map are sequentially searched according to a known algorithm such as "Dijkstra" so as to obtain the shortest distance from a start point to a target point, whereat line segments connecting the start point with the target point are of least cost, for example, in terms of total length (distance) or mean travel time to be required or in combination with other roadway's parameters such as road width, user tax and the like.
It may be considered that a plurality of possible different courses from the start point to the target point are searched one by one priority of smaller cost of line segments therebetween and one of them is finally selected.
However, when the driver does not desire to select the first searched course (shown in FIG. 5) because of, for example, a congested street included therein and he requests finding another course, the apparatus may search a next course that is almost the same as the previous one but has a minimum bypassing as shown in FIG. 6. This, of course, does not meet the driver's requirement.
In case when the driver requests another course from the start point to the target so as to find optimal one with due consideration, for example, of traffic congestion, the apparatus is requested to search an effective course that is different from the previously searched one.
The problem to be solved is that when searching two or more different courses between a start and a target point set on a road map shown a display screen on the basis of digital data of the road map by priority of smaller cost of its total line segments, the conventional apparatus can select only a course substantially similar to the preceding one, omitting different courses from the scope of searching.